Diabetes mellitus is a chronic disease which occurs when the pancreas does not produce enough insulin (Type I), or when the body cannot effectively use the insulin it produces (Type II). This condition typically leads to an increased concentration of glucose in the blood (hyperglycemia), which can cause an array of physiological derangements (such as, for example, kidney failure, skin ulcers, or bleeding into the vitreous of the eye) associated with the deterioration of small blood vessels. Sometimes, a hypoglycemic reaction (low blood sugar) is induced by an inadvertent overdose of insulin, or after a normal dose of insulin or glucose-lowering agent accompanied by extraordinary exercise or insufficient food intake.
Electrochemical sensors are useful in chemistry and medicine to determine the presence or concentration of a biological analyte. Such sensors are useful, for example, to monitor glucose in diabetic patients and lactate during critical care events. A variety of intravascular, transcutaneous and implantable sensors have been developed for continuously detecting and quantifying blood glucose values. Many conventional implantable glucose sensors suffer from complications within the body and provide only short-term or less-than-accurate sensing of blood glucose. Additionally, many conventional transcutaneous sensors have problems in accurately sensing and reporting back glucose or analyte values continuously over extended periods of time due to non-analyte-related signals caused by interfering species or unknown noise-causing events.
Measuring temperature in a sensor environment can be an important aspect of ensuring accurate detection and measurement of analytes for a variety of reasons. For example, changes in temperature are noted as having a corresponding effect on changes in sensor sensitivity. This relationship may be based on a number of factors, including, for example, a change in membrane permeability, or a change in enzyme activity.
Temperature considerations are also important in determining accurate analyte measurements due to the fact that the temperature at which a particular sensor may have been calibrated may be different than the temperature of the sensor's operational environment. Further, because sensor sensitivity changes as temperature changes, it is important to measure the temperature of the sensor environment at or substantially near the time of analyte measurement because the sensor sensitivity may be different than at the time of sensor calibration.
Electrochemical analyte sensors are sensitive to temperature changes because such changes affect enzymatic reaction kinetics. In most patients, homeostatic mechanisms maintain body temperatures within a fairly constant range. Heretofore, the calibration process, wherein a sensor is calibrated at a given temperature, has been relied upon to provide adequate compensation for temperature effects. While reliance on the calibration process may be adequate for sensors placed in areas of tissue that are exposed to relatively small fluctuations in body temperature (such as, for example, subcutaneous adipose tissue in, for example, the abdomen), sensors placed in alternate sites, however, (such as, for example, the dorsal upper arm) may be exposed to greater temperature variations. Similarly, sensor performance may be altered (namely, due to changes in sensor properties, such as sensor sensitivity) when patients are febrile or exposed to large fluctuations in ambient temperatures.